The present invention relates to a novel lock assembly for mounting to a door or an associated door frame, preferably of the type wherein the door will control access to a secured area. More particularly, the invention relates to a novel lock assembly including an elongated latch bolt member for added security, without requiring an additional width or thickness dimension of the latch bolt, of the door, the lock casing, or of the associated door frame.
There are many instances where it is desirable to control access to a secured area by means of a lock mechanism used on the door leading to the area. For example, modern penal institutions of the medium to high security type employ specialized security lock apparatus, often including monitoring and remote control apparatus for cell doors. This is done to provide security for the guards as well as for individual inmates. In this regard, it is necessary to control door operations so as to control the ingress and egress of individual inmates at all times, as well as to prevent possible tampering with and/or breakage of the locking assemblies.
The lock assembly of the invention is adapted for use in a swinging-type door installation. The lock assembly is carried in a casing mountable to one of the door and the door frame, the casing being provided with a face plate. Generally speaking, the face plate and casing may be provided with various tamper proof features for preventing the removal of the lock assembly from the door or door frame other than with specialized tools or the like. The lock assembly for such an installation also includes a latch bolt normally spring biased or otherwise movably biased to extend a portion thereof externally of the face plate for engagement with a strike plate on the associated door or door frame. This strike plate carries a centrally located strike aperture for receiving the extended latch bolt portion therein to achieve locking or latching of the door. However, to reach this locked or latched condition, the latch bolt must first retract to pass the portion of the strike plate surrounding the strike aperture. To this end, this outwardly extending or latching portion of the latch bolt is generally provided with a beveled surface to engage the leading edge surface of the strike plate as the door swings to a closed position so as to effect retraction of the latch bolt against the spring biasing thereof. The beveled surface of the latch bolt is formed at substantially a 45-degree angle so as to provide the force component applied to retract the latch bolt during the foregoing closing procedure.
The maximum thickness dimension of a latch bolt having a 45-degree bevel has heretofore been the same dimension as its "throw", that is, the length dimension of its latching portion. This is necessary to assure that the beveled surface will engage the strike plate during closing. That is, if a latch bolt is provided having a latching portion of greater length than thickness, there will be some portion of the surface thereof to which the 45-degree bevel, which it will be understood begins at the outermost edge or tip of the latching portion, will not extend. Hence, with such an elongated latch bolt it is likely that this non-beveled or substantially flat surface would strike the strike plate, failing to retract the latch bolt and properly close the door. Such engagement might also cause some damage or deformation to the latch bolt, strike plate, or to other internal parts of the locking assembly, or even to the door or door frame.
Notwithstanding the foregoing, in a security installation it is often desirable to provide for some increased length of the throw or the latching portion of the latch bolt. It will be recognized that providing a somewhat longer latch bolt throw adds to the security of the latched or locked door assembly as just described. For example, attempts at tampering with the door or otherwise forcing the door or lock open may be countered by providing some additional length to the latching portion of the latch bolt member. This additional length disposes the beveled portion of the bolt more deeply interiorly of the strike aperture, and makes it difficult for an individual to engage the beveled portion with a tool or pry bar. As previously indicated, such additional length normally requires corresponding additional thickness. However, the additional thickness dimension of the latching portion required to accommodate a 45-degree bevel with an increase in length is not simple to provide in most instances.
In this regard, it is important in security installations to maintain as great a thickness as possible of the door or door frame material holding the strike plate, and especially about the strike plate aperture thereof. This is done to make attempts at tampering with, breaking or otherwise overriding the lock by releasing the latch from the strike plate more difficult. Most installations utilize a one and three-quarter inch thick door. Accordingly, a corresponding width dimension is provided in the portion of the door frame between the jamb and outer edge in cases where the strike plate is mounted in the door frame. In any case, it will be noted that a three-quarter inch wide latch bolt will require a corresponding width of strike aperture, thus leaving one-half inch of material to either side thereof when the strike plate is mounted in the door and a corresponding one-half inch thickness of material to the outside edge of the strike aperture when the strike plate is mounted to the door frame.
Hence, extension of the latching portion of the latch bolt to a length of one inch, while maintaining a 45-degree bevel thereon as described above, would require a one-inch width thereof and hence of the cooperating strike aperture. It will be recognized that this requirement would remove at least one-eighth of an inch of thickness from the material of the door to either side thereof when the strike plate is mounted to the door, or to the outside of the aperture wherein the strike plate is mounted to the door frame. Such reductions of thickness of the material of the door to either side of the strike plate aperture or of the material of the door frame to the outside of the aperture are believed to weaken the installation undesirably in a critical area thereof.
Nonetheless, additional length of the latch bolt is desirable to deal with tampering as mentioned above, and particularly to thwart attempts to override or defeat the lock by an action known as "spreading". In "spreading" a pry bar or other tool is placed between the closed door and door frame and an effort is made to pry the door and door frame apart to a sufficient extent either to release the latch bolt entirely or to expose a sufficient portion of the beveled surface of the latch bolt to an attempt to manually retract said bolt. It will be appreciated that provision of additional length to the outwardly extending or latching portion of the latch bolt reduces the possibility of overriding operation of the lock or otherwise tampering with or breaking the lock by means of such "spreading".